The present invention relates to an apparatus and method for improved thermal conductivity and mechanical support between structures in travelling-wave tubes and, additionally and in combination, for providing shock-resistance and vacuum exhaust in travelling-wave tubes.
In travelling-wave tubes a stream of electrons is caused to interact with a propagating electromagnetic wave in a manner which amplifies the electromagnetic energy. To achieve such interaction, the electromagnetic wave is propagated along a slow-wave structure, or circuit section. The circuit section is housed by a wall in a vacuum environment. A conventional circuit section may include a conductive helix wound about the path of the electron stream or a folded waveguide type of structure. The latter structure also may be known as a coupled cavity or interconnected-cell type. Regardless of its specific configuration, a waveguide is effectively wound back and forth across the path of the electrons. The slow-wave structure provides a path of propagation for the electromagnetic wave which is considerably longer than the axial length of the structure and, hence, the travelling wave may be made to effectively propagate at nearly the velocity of the electron stream. The interactions between the electrons in the stream and the travelling wave cause velocity modulations and bunching of electrons in the stream. The net result may then be a transfer of energy from the electron beam to the wave travelling along the slow-wave structure.
In the coupled-cavity type of slow-wave structure, a series of interaction cells, or cavities, are disposed adjacent to each other sequentially along the axis of the tube. The electron stream passes through each interaction cell, and electromagnetic coupling is provided between each cell and the electron stream. Each interaction cell is also coupled to an adjacent cell by means of a coupling hole at the end wall defining the cell. The travelling-wave energy traverses the length of the tube by entering each interaction cell from one side, crossing the electron stream, and then leaving the cell from the other side, thus travelling a sinuous or serpentine, extended path.
To function properly, such travelling-wave tubes must operate within an acceptable temperature range and, therefore, the heat generated in the circuit section must be removed. Thus, the circuit section must be supported in intimate thermal contact with the vacuum wall by some form of mechanical bond in order to conduct the heat from the circuit section to a heat sink thermally coupled to the vacuum wall.
Conventional thermomechanical bonds may be formed by brazing, heat shrinking, crimping, coining and clamping, as described in U.S. Pat. Nos. 3,268,761 (brazing or spot-welding), 3,540,119 (heat shrinking), 4,712,293 (crimping), 4,712,294 (coining) and 3,514,843 (clamping). A further U.S. Pat. No. 2,943,228 claims a simplified clamp lacking such means for joining parts as welds, brazes, or other metal flow processes. Notwithstanding, under conditions of high heat load, these bonding techniques may contribute to a potential decrease in performance of the travelling-wave tube, for example, by an adverse change in the circuit RF match, in the event that the structure of one or both of the joined elements deform by exertion of pressure from the bond, by stress resulting from changes in temperature, humidity and the environment, or by contamination from braze alloy and the like. Thus, it is desired that any such decreased performance be avoided.